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Discussion Starter · #1 ·
Many of you have probably seen my posts about the mileage I have been getting on my truck before and after I removed the DPF. For those of you that haven't here are my numbers:
DPF on: 16-19 highway, 12-14 city (Not including fuel burned for regen)
DPF off: 17-23 highway, 15 city

My truck had 30k miles on it so it is pretty well broken in for now, and my mileage has been steady for the last several thousand. If you look at the numbers it's clear that the DPF does cause a decrease in fuel economy, but I've got another theory to go along with it.

I'm starting to believe that the sequential turbos have something to do with the poor mileage that these trucks have been getting. It think they are providing too much boost when you don't need it. Here's why I'm thinking this. Today I was driving to my classes with a 15-20 mph wind at my back and I was getting 22.8 mpg and running 6.5-7.5 psi of boost. Now on my way back I was driving into the wind running 12-15 psi of boost getting 17 mpg. And yes I do understand that wind will cause a difference in mpg's but this also happens when there is no wind at all effecting me.

Every once and awhile when I'm driving in a flat area the boost will increase to 12 psi or so and my mileage decreases, for no apparent reason other than the truck decided it wanted more air.

Basically what I'm getting at is this, can there be an economy tune made that reduces the amount of boost made by the turbos at highway speeds? This is just plain curiosity, it would never hurt to know.
 

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Why would more air reduce economy? As long as it doesn't increase fuel inlet it should stay about the same, i think. Thats my understanding of compression ignition anyway.
 

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This may be stupid but can you explain to me what DPF is?
 

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gotcha....not gonna worry about it then.
 

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Why would more air reduce economy? As long as it doesn't increase fuel inlet it should stay about the same, i think. Thats my understanding of compression ignition anyway.
these trucks are boosting the way they are due to some sort of increased load.
but i agree with the op, boost on these kills mileage
 

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This is interesting that this happening. I take it your in 6th gear when this happens? Whats the rpms like when this sudden jump in pressure. I have one other question when you removed the DPF does the exhaust system still do the automatic cleanse option? That is probably not what it is called but I heard that raw diesel fluid is squirted into the exhaust system to clean the exhaust or somehting of that nature (this is the same thing prompting the early recall from flames spewing out while in idle).
 

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I live in mass and encounter the common hilly terrain you associate with new england...

that being said...my truck rarely downshifts...it generally compensates on hills with BOOST...i can run flat around 5 psi...throw in one small hill suddenly im at 15-18 psi and fuel econ is dropping...god forbid you hit a long 5 or 6 percent grade, boost flys way over 25 psi (and fuel economy drops substantially).

All that time..truck will never downshift....im sure a down shift would drop fuel econ too, but just wonder which one is the lesser of the evils...
 

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Always remember to create boost you need to add fuel to spin the turbine, so anytime your boost is up you are providing more fuel, so your mileage will decrease:thumb:
 

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with more boost you do add more air. more fuel is added too in response to the air flow through the maf. otherwise, you would run too lean.
 

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Discussion Starter · #11 ·
This is interesting that this happening. I take it your in 6th gear when this happens? Whats the rpms like when this sudden jump in pressure. I have one other question when you removed the DPF does the exhaust system still do the automatic cleanse option? That is probably not what it is called but I heard that raw diesel fluid is squirted into the exhaust system to clean the exhaust or somehting of that nature (this is the same thing prompting the early recall from flames spewing out while in idle).
Yes I'm all the way in OverDrive gear, and I no longer have any form of a regeneration cycle left on my truck.

But some of you are understanding what I'm saying. More air needs more fuel resulting in less mileage. The boost response on these truck is awesome while your towing or in very hilly areas, and I've never had to down shift in my truck for a hill, the boost just shoots up. But it is very sensitive, any load it feels and here comes the power. I was getting 42 psi stock, and now with the DPF off and tuner I have gotten over 50 psi :icon_ford:

My whole idea is if it's possible to decrease the boost the truck produces while unloaded at highway speeds we should be able to get even better mileage.
 

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Yes I'm all the way in OverDrive gear, and I no longer have any form of a regeneration cycle left on my truck.

But some of you are understanding what I'm saying. More air needs more fuel resulting in less mileage. The boost response on these truck is awesome while your towing or in very hilly areas, and I've never had to down shift in my truck for a hill, the boost just shoots up. But it is very sensitive, any load it feels and here comes the power. I was getting 42 psi stock, and now with the DPF off and tuner I have gotten over 50 psi :icon_ford:

My whole idea is if it's possible to decrease the boost the truck produces while unloaded at highway speeds we should be able to get even better mileage.

OR if you could add something besides fuel when the boost goes up...such as water/meth injection....


I dont think you are going to be able to keep the boost down, I mean thats how the truck makes power, so when you need to accelerate, the boost is going to rise.

I would guess your RPM's were higher while driving into the headwind as well.
 

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I live in mass and encounter the common hilly terrain you associate with new england...

that being said...my truck rarely downshifts...it generally compensates on hills with BOOST...i can run flat around 5 psi...throw in one small hill suddenly im at 15-18 psi and fuel econ is dropping...god forbid you hit a long 5 or 6 percent grade, boost flys way over 25 psi (and fuel economy drops substantially).

All that time..truck will never downshift....im sure a down shift would drop fuel econ too, but just wonder which one is the lesser of the evils...
This is what I was going to say.......yes when the truck senses the its losing any kind of speed and the pedal position is the same it will use the variable viens and increase boost, instead of down shifting or increasing RPMs, increasing boost is not only the most economic way to get more power, but it is less wear and tear on the motor and tranny......also South is right in the keeping the turbos below 10psi is the best way to see good MPG, buts not easy to do :nod:
 

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Discussion Starter · #14 ·
OR if you could add something besides fuel when the boost goes up...such as water/meth injection....


I dont think you are going to be able to keep the boost down, I mean thats how the truck makes power, so when you need to accelerate, the boost is going to rise.

I would guess your RPM's were higher while driving into the headwind as well.
No my rpm's stay the same, 1920 rpm's. The thing is I'm not trying to accelerate, just cruise at the same speed on the highway. But when the truck senses a slight force against it it closes the vanes on the turbo slightly, which increases the boost and that means more fuel is pumped in. What I'm wondering is if a program can be produced that will allow the engine to do the work without the vanes closing on the turbo and increasing boost. If I could keep the boost at 7 psi or lower I could easily stay at 22+mpg. It could just be a fuel economy tune that you wouldn't use when the boost power is required, such as towing or in mountainous terrain.
 

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Your RPMs stay consistent because in a given gear at a given speed your engine will be turning a specific RPM (discounting torque converter slippage). High boost correlates with any need for greater power, which is why boost is higher when you're towing.

When you're driving into the wind, you're effectively loading the truck just like if you were towing something. We've all seen the difference in fuel economy these trucks make at 70mph versus at 85mph, and the reason for that is that the force exerted on your truck is exponentially related to speed (something like speed to the 1.5th power). To go 60mph with a 15mph tailwind entails a drastically different power requirement than to go 60mph with a 15mph headwind.

For a simple experiment, hop on a bicycle and try to maintain 20mph into a headwind and again with a tailwind. After a few miles you'll see how much more tired you are going into the wind! You being tired on a bike is analogous to your truck drinking more fuel.
 

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Great idea............something like Dodge and GM are doing when they drop 4 cylinders when cruising.
 

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Yes I'm all the way in OverDrive gear, and I no longer have any form of a regeneration cycle left on my truck.

But some of you are understanding what I'm saying. More air needs more fuel resulting in less mileage. The boost response on these truck is awesome while your towing or in very hilly areas, and I've never had to down shift in my truck for a hill, the boost just shoots up. But it is very sensitive, any load it feels and here comes the power. I was getting 42 psi stock, and now with the DPF off and tuner I have gotten over 50 psi :icon_ford:

My whole idea is if it's possible to decrease the boost the truck produces while unloaded at highway speeds we should be able to get even better mileage.
Just because boost goes up doesn't mean that there is more fuel being injected...



No my rpm's stay the same, 1920 rpm's. The thing is I'm not trying to accelerate, just cruise at the same speed on the highway. But when the truck senses a slight force against it it closes the vanes on the turbo slightly, which increases the boost and that means more fuel is pumped in. What I'm wondering is if a program can be produced that will allow the engine to do the work without the vanes closing on the turbo and increasing boost. If I could keep the boost at 7 psi or lower I could easily stay at 22+mpg. It could just be a fuel economy tune that you wouldn't use when the boost power is required, such as towing or in mountainous terrain.
The increased boost is in response to increased load. I don't see why you think that running less boost with the same amount of fuel would increase mileage. The turbocharger is driven by exhaust gasses, and while it does cause some loss in terms of exhaust restriction, it generally doesn't affect the engine much. I suppose there are increased "pumping losses" associated with compressing the more dense air (compression stroke), but this alone wouldn't cause the drastic reduction in MPG that you are talking about.

It's true that the engine does inject more air than is needed to ensure that all fuel injected is combusted. The reason for this is that the amount of fuel injected responds quicker to throttle input than the turbochargers and correspondingly the amount of air within the cylinder, which would produce smoke in the form of unburned fuel.

Great idea............something like Dodge and GM are doing when they drop 4 cylinders when cruising.
Not quite the same
 

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Discussion Starter · #18 ·
In my first post I said that even driving on flat roads without any wind at all the boost will level out anywhere between 7-12 psi. This results in a 6mpg difference between those two pressure driving the same flat road with no wind. Some days it will stay at 7psi and I'll get 23mpg, and others it will stay at 12psi and I'll get 17mpg. I always drive the same speed also, so there's no difference there.

I just think that if a custom tune could be made that would make the computer less sensitive to small loads, such as wind and light trailers, the boost could stay low and mpg's would increase.
 

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Adding boost helps the fuel burn more, less smoke. This does not mean that more fuel is not added but if the same amount of fuel is used boost gets more out of it. If the vehicle smokes a lot it needs more boost. If it's running right more fuel is added when boost goes up. Otherwise the boost should have already been up to maximize fuel burn.

When you see the boost go up it is to get enough power to stay where your foot is. Let up and slow down and it will go down. It is a change in load that is causing it. Less turbo would make it use more fuel or it would decrease power.

We can see improved mpg if we slow down up hill and sped up down hill. This is not usually practical and so I just let it Gnaw.

I have also been with a high wind for quite a few miles and saw 22.8 mpg. This is like having a lighter truck, going slower, or going down hill.

Air pressure is exponential. If you double your speed the air forces are increases by eight fold. Weight is one to one. Double the weight and it takes twice the power.

Also when the wind is blowing a twenty mph wind has eight times the power of a ten mph wind.


On another observation I filled up a few days ago and my mpg towing sucked. 8.5 mpg.

I have been seeing 10.5 to 12. I think the trucks don't like some diesel as much as others. It's also possible I have added more weight to my camper than I thought.
 

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Hopefully this clears up some confusion. I've seen a lot of people on here that say the increased boost lowers fuel economy. Wrong. More fuel reduces fuel economy. Here is the best way to explain it. Added fuel is not caused by higher boost, actually higher boost is caused by added fuel.

Ill throw some #'s out there, im sure they are WAY off, but its for example. Lets say you are cruising on a flat road at 65 MPH. Your boost is at 5 PSI and you are injecting 10 CCs of fuel per injection. You come up on a hill, your boost climbs to 10, and now you are injecting 25 CCs of fuel per injection. That is why your econ goes down.

Now, same numbers, different scenario. What some of you seem to think is that if you are goin 65 at 5 PSI and 10 CC/injection, that if you reduce the boost to say 2 PSI, you will get better econ. Actually that would make the econ worse. Ya guys follow what im sayin?

Higher boost is the by product of more fuel burned.
 
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